Atmospheric Chemistryby Ann M Holloway, Richard P Wayne
Atmospheric Chemistry provides readers with a basic knowledge of the chemistry of Earth's atmosphere, and an understanding of the role that chemical transformations play in this vital part of our environment. The composition of the 'natural' atmosphere (troposphere, stratosphere and mesosphere) is described in terms of the physical and chemical cycles that govern
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Atmospheric Chemistry provides readers with a basic knowledge of the chemistry of Earth's atmosphere, and an understanding of the role that chemical transformations play in this vital part of our environment. The composition of the 'natural' atmosphere (troposphere, stratosphere and mesosphere) is described in terms of the physical and chemical cycles that govern the behaviour of the major and the many minor species present, and of the atmospheric lifetimes of those species. An extension of these ideas leads to a discussion of the impacts of Man's activities on the atmosphere, and to an understanding of some of the most important environmental issues of our time. One thread of the book explains how living organisms alter the composition and pressures in the atmosphere, modify temperatures, and change the intensity and wavelength-distribution of light arriving from the Sun. Meanwhile, the living organisms on Earth have depended on these very same environmental conditions being satisfactory for the maintenance and evolution of life. There thus appear to be two-way interactions between life and the atmosphere. Man, just one species of living organism, has developed an unfortunate ability to interfere with the feedbacks that seem to have maintained the atmosphere to be supportive of surface life for more than 3.5 billion years. This book will help chemists to understand the background to the problems that arise from such interference. The structure of the book and the development of the subject deviate somewhat from those usually encountered. Important and recurring concepts are presented in outline first, before more detailed discussions of the atmospheric behaviour of specific chemical species. Examples of such themes are the sources and sinks of trace gases, and their budgets and lifetimes. That is, the emphasis is initially on the principles of the subject, with the finer points emerging at later points in the book, sometimes in several successive chapters. In this way, some of the core material gets repeated exposure, but in new ways and in new contexts. The book is written at a level that makes it accessible to undergraduate chemists, and in a manner that should make it interesting to them. However, the material presented forms a solid base for those who are extending their studies to a higher level, and it will also provide non-specialists with the background to an understanding of Man's several and varied threats to the atmosphere. Well-informed citizens can then better assess measures proposed to prevent or alleviate the potential damage, and policy makers more realistically formulate the necessary controls on a sound scientific foundation.
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By Ann M. Holloway, Richard P. Wayne
The Royal Society of ChemistryCopyright © 2010 Ann M. Holloway and Richard P. Wayne
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1.1 CHEMISTRY IN THE ATMOSPHERE
Our atmosphere is an extraordinary mixture of gases and suspended particles, some inert and some highly reactive, some present in large quantities while others are found only in the minutest traces. New material is being added continuously to the atmosphere at the surface boundary, with materials trapped within the Earth, perhaps when the planet was formed, being liberated, sometimes slowly and gently, and sometimes violently in volcanic eruptions. So far, this description would also fit the atmospheres of our near neighbours Venus and Mars. But on Earth, the living organisms (the biota) make a quite dramatic contribution of their own to the supply of chemicals to the atmosphere. Humans are part of the biota, and are making an impact on the atmosphere out of all proportion to human life's biological importance, as we shall see time and again in this book. The study of the chemistry of the 'natural' atmosphere is truly fascinating in its own right. But it is also clear that the chemist who has a good understanding of how this natural atmosphere works is one of the most likely candidates to make rational and informed suggestions about ways to offset Man's depredations. Policy makers (and politicians) need chemists!
The mixture of gases and particles naturally contains many substances that can react with others that are present, and one of the primary tasks of the atmospheric chemist is to interpret the composition of atmosphere in terms of the pathways and rates of the reactions that occur. A knowledge, preferably obtained from experimental observations, of the possible mechanisms and of the chemical kinetics of the reactions thus lies at the core of this aspect of atmospheric chemistry. It is often helpful to imagine the atmosphere as a giant chemical reactor in which the chemical soup interacts to remove some components and to replace them with new species.
Of the biogenic gases, the most notable both in abundance and in properties is oxygen, O2, produced by photosynthesis. The existence of life allows oxygen to make up about one-fifth (21 per cent) of Earth's atmosphere, while oxygen is almost absent from the atmospheres of Venus and Mars. However, many of the other gases released are reduced (CH4, H2S, (CH3)2S, for example), or at least only partially oxidized (CO and NO will serve as examples). Much of the chemical change in the lower part of the atmosphere thus consists of oxidation steps that lead ultimately to CO2, H2O, SO3, NO2 and N2O5 for the examples presented. We will look again at these products quite soon. For the moment, the significant feature to note is that the atmospheric conversion steps are driven, directly or indirectly, by solar ultraviolet radiation, so that the atmosphere is not just a reactor, but a photochemical reactor. To be sure, chemistry continues at night: charged particles from the Sun and lightning discharges provide energy for some reactions, while others can be promoted thermally near volcanoes. But by far the greatest proportion of the chemistry that we have outlined so far is ultimately dependent on photons emitted by the Sun.
This apparently simple picture of chemistry in the lower part of the atmosphere not surprisingly reveals considerable complexity when looked at more closely. It is at this point that the very close interactions between chemical, physical, biological and geological processes become evident. Chemical change is greatly influenced by temperature and pressure; not only the rates of reactions, but even the pathways open for reaction and thus the products formed may depend on these two factors. As a result, in the atmosphere altitude, latitude and longitude may all play a part in determining which processes occur, or at least compete successfully with alternative pathways. Pressures and temperatures in the atmosphere fairly obviously change with altitude, although the behaviour turns out to be far from obvious. It is here that we encounter the concept of different regions of the atmosphere, such as the troposphere, the stratosphere and the mesosphere, in which the classification is based on temperature structure. Figure 1.1 shows where the regions are located. Altitudes z are shown on the left-hand vertical scales, and temperatures T are plotted horizontally; the line shows how T varies with z. Atmospheric motions transport substances that live long enough to other heights and places. Reactions take place on the surfaces of suspended solids (aerosols) such as ice, soot and dusts, and within droplets (liquid H2O clouds, for example), as well as just within the gas phase, so that physics and meteorology are necessarily again involved in a detailed description of atmospheric chemistry. But chemical composition, and chemistry itself, may have an effect on temperatures, so there is a reverse interaction. Biology makes many of the chemicals released to the atmosphere, but life is sustained or shielded in one way or another by the atmosphere. Geology and geochemistry influence the chemical composition of the atmosphere, and in turn are influenced by it. It is worth noting, too, how the interactions often run in two directions: temperature affects chemistry, but chemistry influences temperatures. These so-called feedbacks thus need to be taken into account when attempting to understand atmospheric behaviour. Indeed, we do not get to a very deep understanding without considering the feedbacks between all the systems taken as a whole. It's a complex and daunting task, but a fascinating and scientifically stimulating one that provides insights at all sorts of levels.
1.2 EARTH'S ATMOSPHERE IN PERSPECTIVE
Although we shall consider many more of the effects that life has had on Earth's atmosphere later in the book, it is interesting to examine the pie charts in Figure 1.2, which indicate the actual compositions of the atmospheres of Venus, Earth and Mars. Comparison of the chart for Earth with the charts for Venus and Mars, the planets on either side of us, will show just how dramatically the existence of life on Earth has altered the composition of our atmosphere, particularly with respect to the proportion of carbon dioxide in the atmosphere.
At this point, the unexpected nature of the Earth's atmosphere becomes apparent. Since Earth lies in the solar system between Venus and Mars, Earth's atmosphere might have been expected to consist primarily of the oxidized compound, carbon dioxide. But CO2 is only a minor (although very important) constituent. The presence of elemental oxygen as a major constituent is obviously one of the most significant features of our atmosphere and has some of the greatest impacts on its chemistry. Earth's atmosphere appears to be a combustible mixture, since there is too much oxygen in the presence of too many gases that react with oxygen. Oxygen reacts with hydrogen to form water, with nitrogen to form nitrates, with methane to form carbon dioxide and water, and so on. Biological processes are dominant in the production of the oxidizable components of our atmosphere, and Earth's atmosphere consequently maintains a steady-state disequilibrium composition.Furthermore, the entropy of the atmosphere is effectively reduced — the energy required for this reduction is supplied almost entirely by radiation from the Sun, which enables biology to produce both oxidizable components and, of course, oxygen.
As any high-altitude mountaineer will attest, most of this oxygen is found close to the surface of the Earth, and the atmosphere loses density very quickly as an ascent is made. Gaseous components in the atmosphere do not settle down on the planetary surface under the influence of gravitational attraction because the translational kinetic energy of the atoms or molecules competes with the forces of sedimentation. As a result of this competition, the density of gas falls with increasing altitude in the atmosphere. For the bulk liquids and solids of the oceans and land, the masses of the individual 'particles' are so great that this competition is completely ineffective, although if the material is finely divided enough it can remain in atmospheric suspension (an aerosol: see Section 1.3).
The total mass of Earth's atmosphere is around 5×1018 kg. Half of this mass lies below an altitude of about 5.5 km and 99 per cent of that mass can be found below roughly 30 km, although several atmospheric species may still be found some 160 km or so above sea level, and ions of atomic and molecular hydrogen, nitrogen and oxygen can be found many hundreds of kilometres from the surface of the Earth. However, where the outer 'boundary' of the atmosphere actually occurs is extremely difficult to define; at an altitude of 100 km the pressure is only about one millionth of that at sea-level and the average distance between collisions of the atoms and molecules (the mean free path, λ), has reached 1 mm, several million times greater than the diameters of the colliding partners. At this altitude, chemical reactions are already rather slow because the reactants have such a low chance of colliding with each other. By the time we reach 160 km above the surface, the probability of collisions is very low indeed, and the mean free path is around 100 m.
Our high-altitude mountaineer will have also discovered that, like the density, the temperature decreases as we climb higher. In fact, it drops by about 6 K for every additional kilometre of altitude even for the highest mountains (Everest's peak is at 8848 m and temperatures of 230–240 K are typical at that altitude). However, an unexpected feature of the Earth's atmosphere is that beyond about 15 km altitude the temperature in reality begins to increase. Now is the time to look at Figure 1.1 again. Below this level lies the troposphere (with the final kilometre or so before the surface making up a boundary layer). This is the atmospheric region in which we live: we will discuss it in detail in Chapter 8. For now, we observe that since this lowest part of the atmosphere has higher temperatures towards the bottom, it is subject to convection currents and is characterized by rather strong vertical mixing. It is this behaviour that gives rise to the name of the region, since the Greek for 'turning' is 'tropos'. Above about 15 km altitude, temperatures start to rise, with the consequence that there is very little convection or vertical mixing in this region. The atmosphere becomes 'layered', and is called the stratosphere (Greek 'stratos' = 'layered').
To illustrate just how different these rates of mixing are, we may like to note here that individual molecules usually take several years to travel through the stratosphere, but that they can traverse the entire depth of the troposphere in a few days, or even a few minutes in the updraughts of large thunderstorms. The height at which the boundary between the troposphere and stratosphere (the tropopause) occurs is also variable and depends on season and latitude.
In fact, the temperature structure of the atmosphere as a whole is even more complex than this brief discussion would suggest. As illustrated in Figure 1.1, beyond roughly 50 km, atmospheric temperature once again decreases, the turning point (the stratopause) marking the limit of the stratosphere. We are now entering the mesosphere; temperature reaches its minimum at the top of that region before rising again in the thermosphere, as shown in Figure 1.1.
We shall return later to the questions of how and why the temperatures vary in this way and why the pressures drop with increasing altitude. For the time being, it is just useful to introduce these key concepts, and to remind ourselves that most living organisms are found near the surface. The lowest parts of the atmosphere (the troposphere) receive the greatest variety of chemical species arriving geologically and biologically from the planet's surface; but, because solar radiation has had to pass through the atmosphere that lies above, contribute the lowest-energy photochemistry. In general, then, the chemistry becomes both more energetic and involves simpler species the higher the altitude. At the greatest altitudes, neutral species can be split into atoms, and the atoms and molecules can become ionized.
1.3 AEROSOLS, PARTICLES AND DROPLETS
Most of the mass of the atmosphere is made up of gaseous elements and compounds, and a large part of the chemistry that we shall be exploring in this book naturally concerns gas-phase reactants and reactions. However, there is a surprisingly large amount of non-gaseous material in the atmosphere. Such material includes sea-spray from the ocean, windblown dust from surface erosion, particles from forest fires and volcanic emissions, and meteoric debris. Some chemical reactions that occur between gaseous components in the atmosphere can also lead to particle formation. Even in the unpolluted atmosphere, these processes lead to the haze seen in places such as the Smoky Mountains of the USA. In addition, human activity generates further particulate material through combustion of fuels, and this material has a significant impact on the chemistry of the atmosphere, as we shall see later in the book. At present, mankind probably generates about 20 per cent of all solid particles currently being added to the atmospheric mix. Chemical reactions can occur on the surface of non-gaseous particles or within the body of liquid droplets, and these processes thus add to the rich chemistry of the atmosphere. Although to a large extent these non-gaseous species are to be found in the lower part of the atmosphere (say up to an altitude of 10 km), some are found much higher up. Polar stratospheric clouds, which we will meet in Section 9.4.4, are found in the stratosphere (obviously), and meteors, fragments of asteroids, cometary dust and the like enter the atmosphere at its outermost, indistinct, boundary. All these particles have been found to exert important influences on chemical behaviour. This may be a good point to say that the purely gas-phase processes are called homogeneous and those involving some phase (liquid, solid) in addition to gas are called heterogeneous.
Several different types of non-gaseous atmospheric particles exist. Material may be in liquid or solid forms and varies from the extremely small (around 0.05 µm — although, as we shall see, this end of the scale is very difficult to define) to the rather sizeable (around 1000 µm [equivalent] 1 mm). Suspensions of particles in a gas are called aerosols. In principle, if the particles are liquid we refer to the aerosol as a cloud or mist, while if they are solid, the aerosol is called smoke or dust. However, we need, at this point, to define what we mean by 'suspension'. An aerosol particle in the atmosphere is subject to the competing physical influences of gravity on the one hand and the kinetic energy of surrounding particles on the other, just as are the atoms or molecules of a gas. Gravity constantly pulls the particle downwards and tries to cause sedimentation, whereas the motion of the surrounding molecules means that the particle is subject to drag, which stops it falling. Whether or not the particle remains in the atmosphere (in 'suspension') or falls to the surface of the Earth is largely dependent upon its size. For example, a typical cloud droplet of 10 µm diameter would take a day to fall through a cloud 1 km thick and is, for practical purposes, permanently suspended. However, a raindrop of 2000 µm diameter would fall through the same cloud in under three minutes and is certainly not in suspension, as anyone getting wet as a result of its fall would testify. Table 1.1 shows terminal fall velocities for several sizes of water droplet. When they grow large enough, particles of water in clouds become precipitation in the form of rain, hail, snow and so on. Condensation of water vapour to form droplets usually starts by nucleation on foreign solid-aerosol particles, which act as cloud-condensation nuclei (CCN). Hygroscopic or soluble nuclei are particularly effective CCNs, so their presence in the atmosphere must be regarded as an important component of the evaporation–condensation–precipitation cycle of the Earth's water.
At the other end of the scale there is an imperceptible merging between what is a small aerosol and what is a large molecule or cluster of molecules. The smallest solid aerosols (those with radii less than about 0.5 µm) are known as Aitken nuclei and their concentration in the atmosphere is significantly affected by human activity. Typical counts of Aitken nuclei near the Earth's surface are 105, 104, and 103 particles cm-3 over cities, rural areas and the sea, respectively. We shall discuss these particles further in Section 8.6 and in Chapter 11. For the moment, we simply note that these aerosols have a significant effect on the chemistry of the atmosphere. Clouds and other aerosols may also modify the atmospheric balance of incoming and outgoing radiation and alter atmospheric and surface temperatures by changing both the reflectivity and absorptivity of the atmosphere towards incoming solar radiation. The effects of the eruption of Mount Pinatubo in the Philippines in June 1991 provided an interesting demonstration of climatic effects. Vast quantities of dust and SO2 were injected into the atmosphere, resulting in significantly increased aerosol concentrations, including those of droplets of H2SO4. Furthermore, solar radiation reaching the lower atmosphere was reduced by about two per cent and global average temperatures may have been reduced by as much as 0.5 °C during the year following the eruption. As chemists, we should be alerted to these effects of particulate material in the atmosphere and to the possibility of aerosols altering the course or rates of chemical change. We shall consider the sources of particles and their roles in chemical change throughout the book, and especially in Sections 8.6, 9.4.4 and 11.1. At this stage, we just summarize in Figure 1.3 the sources, lifetimes and effects of particles of different sizes.
Excerpted from Atmospheric Chemistry by Ann M. Holloway, Richard P. Wayne. Copyright © 2010 Ann M. Holloway and Richard P. Wayne. Excerpted by permission of The Royal Society of Chemistry.
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Richard P Wayne is Emeritus Professor of Chemistry at the University of Oxford, and has been a Lecturer or Professor since 1965. Until 2006, he was Dr Lee's Reader in Chemistry as well as Senior Science Tutor at Christ Church, Oxford. He has devised and delivered courses and lectures on atmospheric chemistry at a variety of levels in Oxford, other UK Universities, and in other countries such as France, Germany and Argentina. He is the author of the book Chemistry of Atmospheres (OUP), now in its third edition which, with its multidisciplinary and pedagogic approach is widely regarded as the leading text on the subject. He was also the Founding Editor and Editor-in-Chief 1970-2005 of the Journal of Photochemistry and Photobiology. Ann M Holloway also works at the University of Oxford. Originally an Oxford chemist, she subsequently studied music at London, distinguishing herself by obtaining the top First of her year. She has, in addition, qualifications as a teacher and in mathematics. Her expertise in atmospheric chemistry and her experience in the capabilities and needs of students at all levels are in large part responsible for the structure of the book and for the selection of topics presented.
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